It is proposed to raise antibodies in rabbits against purified Factor IX. This material, obtained by methods described previously, is of high biologic purity, containing non-activated factor IX, and no other coagulation factors. The antibodies will be used for the following experimental procedures: A. To determine antigenic changes taking place during proteolytic activation of factor IX. Factor IX will be activated with trypsin, pronase and thrombin. Aliquots obtained at 2,5 and 10 minutes which contain activated forms of factor IX, will be studied by immunodiffusion and immunoelectrophoresis using the monospecific antibodies to the intact factor IX. B. To develop specific immunoassay for factor IX, in order to survey patients with factor IX deficiency. The concentration of factor IX will be determined for normals and compared to values found in factor IX congenitally deficient patients, in carriers, and in patients with liver disease or on oral anticoagulants. Radioimmunoassays will be used in parallel to coagulation tests in order to establish correlations between coagulant and antigenic factor IX. C. To study the effect on circulating factor IX of therapeutic transfusions with plasma or factor IX concentrates in patients deficient in factor IX, in order to compare changes in coagulant activity to those in antigenic material for various periods of time.